Regeneration of spent ammonium persulfate etching solutions



United States PatentN O REGENERATION OF SPENT AMMONIUM PERSULFATEETCHING SOLUTIONS Kenneth J. Radimer, Little Falls, and Frank E.Caropreso,

Hamilton, NJ., assignors to FMC Corporation, New

York, N.Y., a corporation of Delaware Continuation-impart of applicationSer. No. 451,635, Apr. 28, 1965. This application Oct. 1, 1965, Ser. No.495,374

23 Claims. (Cl. 204-82) This invention relates to the recovery of unusedpersulfate values in spent etching solutions, and further, to theregeneration of spent ammonium persulfate etching solutions.

This application is a continuation-in-part of patent application SerialNo. 451,635, led Apr. 28, 1965, and now abandoned, in the names of'FrankE. Caropreso, Kenneth J. Radimer and Bernard I. lHogya..

Solutions of peroxygen chemicals such as ammonium persulfate arecommonly used to dissolve metals such as copper, cobalt, iron, nickeland zinc. This is desirable, for example, in place of ordinary machiningin order to remove specified amounts of these metals from surfaces offragile or peculiarly shaped objects. A more widespread application ofthis technique is the production of printed electrical circuits. In thisapplication a resist or mask in the form of the desired circuit isplaced over the surface of a copper iilm laminated to a base, and thepartially masked copper film is treated with the etchant. The copperarea not covered by the resist is dissolved, while the copper covered bythe resist remains to form the desired circuit.

Ammonium persulfate solutions are desirable in such applications becausethey do not generate obnoxious fumes, are easy to work with, and arerelatively noncorrosive to certain common materials of construction suchas stainless steels. IIn use, the metal, e.g., copper is dissolved inthe nonmasked areas by the persulfate solution until the dissolutionrate is below commercially acceptable rates. The resulting spent etchantmust be treated to remove the dissolved metal, e.g., copper before it issewered.

One serious problem that has arisen in using this process is thatsubstantial amounts of persulfate are discarded in the spent etchant. Ithas not been possible to recover or make use of these remainingpersulfate values in the spent etchant on a commercial scale; the mereaddition of fresh ammonium persulate to a spent etching solution torestore the original persulfate concentration does not yield an-acceptable etching solution because such solutions give inferior anderratic etching.

A second problem is that the treatment of spent persulfate solutions toremove metal values, e.g., copper, entails an additional process step'which adds to the expense of disposing of these solutions. Copper mustbe removed from the persulfate solutions before they are sewered becauseof the toxicity of the copper values.

One method of eliminating the expense of disposing of spent etchingsolutions is set forth in our parent application Serial No. 451,635wherein said solutions are cooled to a temperature sutiicient tocrystallize copper sulfate values and ammonium sulfate values from thesolution without crystallizing substantial amounts of ammoniumpersulfate; the crystallized solids can be separated from the remainingsolutions which then can be used for etching. This parent applicationalso provides for adding additional ammonium persulfate values to thereactivated solution when desired. This process is helpful in solvingthe waste disposal problem and in utilizing residual persulfate valuesin the spent etchant, but it does not set forth a technique forregenerating persulfate from the remaining values in the spent solution.

3,406,108 Patented Oct. 15, 1968 ice As a result there has been a needfor a method of treating spent persulfate etching solutions on acommercially acceptable basis to recover the residual persulfate values,to regenerate additional persulfate values andto eliminate the disposalproblems associated with spent persulfate solutions.

It is an object of the present invention to treat spent amrloniumpersulfate etching solutions to satisfy these nee s. i

It is a--further object of the present invention to obviate thesediiculties by treating spent persulfate etching solutions to remove themetal values, e.g., copper,`present in the etching solution and toregenerate the persulfate values in said etching solution so as toobtain etchants yielding consistent, commercially acceptable etchingrates.

We have found that a spent aqueous ammonium persulfate etching solutionwhich has been used to dissolve a metal such as copper, cobalt, iron,nickel and zinc, and which contains the corresponding metal sulfate,arnmonium sulfate and residual ammonium persulfate values can beregenerated by treating said solution to remove a substantiallypersulfate-free mixture containing ammonium sulfate and thecorresponding metal sulfate, using the persulfate-free mixture as thecatholyte of an electrolytic cell, using the remainder of said `spentsolution containing sulfate values and residual persulfate values `asthe anolyte of said electrolytic cell, the cathode and anode sections ofthe electrolytic cell being contained in a manner (preferably separatedby a diaphragm) which permits at least hydrogen ions to pass freelybetween the anolyte and catholyte but which prevents any substantialamounts of persulfate in the anolyte from mixing with the catholyte,passing an electric current through said catholyte and anolyte by meansof a cathode in said catholyte and an anode in said anolyte, platingoutl said metal at said cathode, producing persulfate values at saidanode, obtaining a catholyte substantially reduced in metal content, andrecovering an anolyte solution having an increased persulfate contentand suitable as a regenerated etching solution;

It has Afurther been lfound that regeneration of the persulfate valuesat the anode is facilitatedby carrying out the regeneration in thepresence of urea or ammonium thiocyanate (-NH4CNS) in the anolyte.

In the following detailed description of the invention, the process willbe described with reference to regenerating ammonium persulfatesolutions which have been used to etch copper. However, it should beunderstood that copper is only illustrative of one embodiment and thatsubstantially the same effect can be obtained with cobalt, iron, nickelor zinc.

In carrying out the present invention a fresh aqueous 0,75 to 1.25 molar(M) ammonium persulfate etching solution (containing from about 171 g.to 285 lg. of ammonium persulfate per liter of water) can be heated totemperatures of from about 35 to 46 C. and used to etch unmaskedportions of copper. The etching can take place either by conventionalimmersion etching or spray etching. In the immersion etching process themasked copper workpiece is immersed in the solution for the amount oftime required to etch the exposed copper surface. In the spray etchingtechnique the persulfate solution is discharged from a spray nozzleunder pressure and the spray impinges on the masked copper workpiece.

In practice, the spray etching technique is preferred because it permitsshorter etching times and results in a better quality etch. This is duein large measure to the constant replacement of the copper-rich layer ofetchant in immediate contact with the workpiece with fresh etchant.

Etching can be continued until the solution has been depleted ofpersulfate values-to va concentration of about 0.4 to 0.6 M ammoniumpersulfate. At this point, the `solution is capable of further etching,but the etch rate and quality of etch diminishes and such solutions arenormally discarded as spent solutions.

In accordance with a preferred embodiment, this spent Vsolution ispassed to a crystallizer, preferably a vacuum crystallizer, which ismaintained ata temperature of to 20 C., whereupon copper sulfate andarnmonium sulfate crystallize leaving substantially all the persulfatevalues in the remaining mother liquor. The exact temperature of coolingis not critical but it should be 'suiciently low to crystallizesubstantial amounts of copper sulfate and ammonium sulfate, normally inthe form of a hydrated double salt, but not ammonium persulfate values.A temperature of about 0 to l0" C. is optimum. In many cases vacuumcrystallizers are desired in order to remove some of the water in theremaining solution.

The'resulting crystals are separated from the solution by filtering orby centrifugal separators. During the separation care must be taken toremove adhered mother liquor from the precipitate in order to preventloss of ammonium persulfate values. The precipitated crystals are thendissolved in water and passed into the cathode section of anelectrolytic cell for use as a catholyte. It is critical that thecatholyte be free of any substantial amounts of persulfate since thepresence of persulfate interferes with the proper functioning of theelectrolytic cell.

The mother liquor from the crystallization step which contains theremaining copper, ammonium and sulfate values as well as the residualpersulfate values, is passed into the anode section of the aboveelectrolytic cell and 4used as an anolyte. The term sulfate values asused in the specification and claims refers to both sulfate andbisulfate water-soluble compounds.

The anolyte and catholyte solutions must be contained in a manner toprevent any substantial amounts of persulfate values in the anolyte frommixing with the catholyte, but without preventing the ow of an electriccurrent between these solutions. Separation of the solutions bydifferences in specific gravity is workable but best results areachieved by placing a diaphragm between the two solutions. The diaphragmpermits the ow of an electric current between the anolyte and catholyte,but prevents substantial amounts of persulfate in the anolyte fromdiffusing into the catholyte. The flow of current between the anolyteand catholyte takes place by passing certain ions, principally hydrogenions, through the diaphragm and is a necessary part of the electriccircuit of the cell. The diaphragm in the electrolytic cell can be anyporous membrane such as a porous porcelain sheet, asbestos membrane,plastic membranes, etc.

Au electrode then is immersed in each catholyte and anolyte solution.These electrodes can be any material which can conduct an electriccurrent and which does not react with the solutions of the cell duringthe electrolysis. Generally, noble metals such as platinum or gold arepreferred as anodes, 4and copper is preferred as the cathode, sincecopper plates out on the cathode in the ensuing reaction. An electricpotential is placed across the electrodes (the anode and cathode) bymeans of a battery, rectifier or other source of direct current tocomplete the electrolytic cell.

The electric potential which is applied across the electrodes must besuicient to cause a positive electrical current to flow outside the cellfrom cathode to anode to plate out copper at the cathode and convertsulfate valuesV to persulfate iat the anode. Normally an EMF of at leastabout 2.5 volts has been found operable with from about 5 to 7 voltsbeing preferred. The electrolytic regeneration is conducted at atemperature of from about 2 to 30 C. with temperatures from about 5 to20 C. being pre- (NHiCNS) is present in the anolyte in concentrations ofup to about 0.2%. The presence `of urea or NH4CNS in vamounts of about0.005% has been found most effective in increasing anode eiliciencyforithe production of persulfate from sulfate by to 100%. The presenceof urea or NH.,CNS in the catholyte does not interfere in any mannerwith the regeneration or operation of the electrolytic cell.

In general, a cathode having a large surface area is used to facilitatecopper deposition while the anode can have a much smaller surface area.Thus, in typical operations, current densities from 25 to 30 amps persquare foot at the cathode and 1100 amps/sq. ft. at the anode have beenused with good effect.

The desired half-reactions for each of the half cells are as follows:

At the cathode- Cu+++2 electrons Cu At the anode- 2SO4=2 electrons S208:The desired overall reaction of the electrolytic cell is 2SO4=+Cu+ Cu+S2O8= In the above described regeneration, current efficiencies forproduction of persulfate at the anode are irnproved by the presence ofhigh concentrations of sulfate values in the anolyte. Normally, the celldiaphragm does not permit ready transfer of sulfate values fromcatholyte to anolyte to replace sulfate values which are converted topersulfate. To provide enough sulfate values to the anolyte for theproduction of high concentrations of persulfate it is desirable tointroduce additional sulfate values into the anolyte in order to augmentexisting sulfate values. This can be done by introducing theseadditional sulfate values directly into the spent etchant before copperammonium sulfate is crystallized from it. Such added sulfate values mustbe capable of passing into the mother liquor during the crystallizationstep.

Ammonium bisulfate is preferred as a source of supplementary sulfatevalues because it does not introduce foreign ions into the anolytesolution. Other soluble compounds which increase the anolytes sulfateconcentration can also be used providing that they do not interfere withthe electrolytic regeneration or with the subsequent etching. Sulfatevalues might be increased by introducing materials such as ammoniumsulfate, sodium sulfate, or sulfuric acid.

The supplementary sulfate values are preferably introduced into thespent etchant before crystallization of copper and ammonium sulfate fromit, as set forth above, in order to increase the extent ofcrystallization of copper values from the aqueous solution; thissolution after being electrolyzed has therefore a lower copperconcentration Which leads to better etching. Alternatively, the addedsulfate values can be introduced directly into the mother liquorsubsequent to crystallization. In either case the sulfate values in themother liquor, which pass into the anolyte, are increased.

In general, one continues electrolysis at least until copper iscompletely removed from the catholyte. This removal of copper isaccompanied by a transfer of hydrogen ions through the membrane fromanolyte to catholyte, hydrogen ions having the highest mobility of anyof the dissolved species, and the catholyte thus becomes substantially asolution of ammonium bisulfate. Electrolysis of the catholyte beyondthis point does not change its chemical composition greatly since anyhydrogen ions which are reduced at the cathode are replaced by hydrogenions diffusing through the membrane.

The electrolysis can be conducted batchwise or continuously. In acontinuous operation electrolytic cells should be used in which theelectrolytes are introduced at one end of the cell and flow along themembranes until they are removed from the cell at the opposite end.During this ow reduction of the copper concentration in the catholyteand an increase in the persulfate concentration in the anolyte occur.The solutions can be introduced into other such cells in cascade ifdesired.

When regeneration has been completed the persulfaterich anolyte ispassed to the etching machine and the eXit catholyte is passed to anevaporator. In the course of the electrolysis the persulfateconcentration of the anolyte can be raised to any extent desired up toabout 0.8 to 1.0 M; such solutions have been found to be acceptableetchants. In the evaporator, water is stripped from the exit catholyte.The resulting -concentrated catholyte contains sulfate values,principally as ammonium bisulfate, and is useful for supplyingsupplementary sulfate values required in subsequent anolytes asdescribed above. In any event the nal catholyte contains virtually nocopper and therefore can be sewered if the ammonium bisulfate values arenot desired.

In general it is preferred to continue electrolysis until all the copperhas been completely plated at the cathode and to operate the cell withanolyte concentrations such that an amount of persulfate, equivalent tothe amount of plated copper, is produced in the same period. It hasoccasionally been found that it is desirable to continue electrolysissomewhat beyond the point at which all of the copper at the catholytehas been plated out in order to produce the desired persulfateconcentration in the anolyte.

In the above description of the invention a crystallization procedurewas used in order to prepare a persulfatefree catholyte. As an alternateto crystallization, a portion of the etchant intended for use ascatholyte may be treated to decompose persulfate values. This may bedone for example by merely heating that portion of the etchant to atemperature between about 50 C. and 100 C. at which temperature thepersulfate decomposes. Alternately, the residual persulfate in thisportion of the etchant may be destroyed by reacting it with a chemicalreducing agent such as sulfur dioxide. The resulting etching solution,free of residual persulfate, can then be used as the catholyte.Untreated spent etchant, which still contains residual persulfate, maybe used as the anolyte. The electrolytic regeneration is carried outusing these solutions as catholyte and anolyte, respectively, in thesame manner as set forth previously.

ln the above description of the invention the desirability of addingsupplementary sulfate values to the anolyte to improve anode eiciency inproducing persulfate has been described. These sulfate values,preferably in the form of ammonium bisulfate can be supplied from anyavailable source including the exit catholyte, once cyclic regenerationhas commenced. However, to start up the process such supplementarysulfate values can be obtained in the form of copper ammonium sulfatecrystals. These can be obtained by cooling some spent etchant to 0 to 20C. to precipitate these crystals, and adding the crystals to the anodechamber during electrolysis.

An alternate method is to manufacture ammonium bisulfate for use instart up. One method is to cool spent etchant to obtain crystals,redissolve these crystals in water for use as a catholyte, and use themother liquor as the anolyte. Upon electrolysis of this cell, ammoniumbisulfate is formed in the catholyte which can be used as a source ofsupplementary sulfate values.

One embodiment of the invention that is contemplated to enable anammonium persulfate solution to be used indefinitely for etching iscarried out by continuously removing a portion of the etching solution,continuously add-ing concentrated exit catholyte thereto, cooling theresulting solution to precipitate sulfate insolubles and continuouslyremoving sulfate insolubles from the supernatant solution. The sulfateinsolubles (free of persulfate) are dissolved in water continuously andthis solution continuously fed to the cathode compartment of anoperating electrolytic cell; the remaining supernatant solut-ion insimilar fashion is fed continuously in the anode compartment of theelectrolytic cell. The resulting electrolytic regeneration increases thepersulfate values in the anode compartment of the electrolytic cellwhile converting copper ions to copper metal in the cathode compartment.The regenerated anolyte, enriched in persulfate values, is removed fromthe cell and recycled back to the main body of etching solutioncontinuously. The exit catholyte, substantially free of copper, ispassed continuously into an evaporator to remove some of its water. Theresulting concentrated exit catholyte is removed and continuously addedto the stream of spent etchant prior to the cooling and crystallizingstep as described above.

The resulting etching solution in such a process thus can be usedcontinuously with this cyclic crystallization and regeneration procedureunless or until foreign impurities which may be introduced inadvertentlyor build up in the solution to a point where the quality of etch isaffected. Such a process obviates the need for replacing etch-ingsolution and for disposing of the spent solutions or other Wastematerials.

In general the quality of etch obtained by using regenerated persulfatesolution is about the same as that obtained with fresh ammoniumpersulfate solution. Thus the electrolytic regeneration techniquedescribed above does not entail any reduction in the high quality ofetch obtained when using ammonium persulfate etchants.

The present invention will now be described by reference to the drawingywhich is a diagrammatical flow sheet of the process.

In the drawing an ammonium persulfate etching solution is heated in anetching machine 2 and used to etch photo-resisted, copper laminatedprinted circuit panels un. til the rate and quality of etch are nolonger commercially acceptable. The spent solution is then withdrawnfrom the etching machine 2 and transferred to a vacuum crystallizer 4 inwhich the contents are cooled to a temperature of 0 to 10 C. In thevacuum crystallizer 4 some Water is vaporized during cooling and sent tocondenser 24; during cooling crystals of copper ammonium sulfate areprecipitated. These crystals, which contain substantially no ammoniumpersulfate, are separated from the mother liquor in centrifuge 6. Themother liquor which contains copper sulfate, ammonium sulfate, ammoniumbisulfate and residual ammonium persulfate is then discharged into theanode compartment 8 of an electrolytic diaphragm cell 10 for use as theanolyte. The copper ammonium sulfate crystals are dissolved in waterobtained from water storage tank 26 and discharged into the cathodecompartment 12 of the diaphragm cell 10. The diaphragm cell 10 is madeup of an anode compartment 8 and a cathode compartment 12 separated by adiaphragm 14 through which hydrogen ions can pass, but which preventsany substantial diffusion of other ions from one side of the diaphragmto the other.-The diaphragm must prevent substantial amounts ofpersulfate ions from diffusing from the anode to the cathodecompartment. Where desired, the free surface of the catholyte in thecathode compartment 12 can be maintained slightly higher than that ofthe anolyte in the anode compartment 8 to diminish such diffus-ion. Theanolyte and catholyte are electrolyzed by passing a direct currentthrough the anode 16 immersed in the anolyte and through the cathode 18immersed in the catholyte. The direct current is supplied by battery 20or other equivalent source of direct current. Within the cell electricalcurrent is carried between the 7 anode and cathode by the ions whichdiffuse through the diaphragm.

During electrolysis, copper ions in the catholyte are converted tometallic copper' and are plated out on the cathode 18. Sulfate values inthe anolyte simultaneously are converted at the anode 16 intopersulfatevions. The

diaphragm cell 10 is properly operating when the quantity of copperplated out at the cathode 18 is approximately equivalent chemically tothe amount of persulfate produced in the anolyte. Electrolysis iscontinued until all the copper from the catholyte has been platedout oncathode 18 and the desired concentration of persulfate has been reachedin the anolyte compartment S. The exit anolyte, rich in persulfate, isrecycled back to the etching machine 2 for further etching. The exitcatholtye from cathode compartment 12, which contains virtual- 1y nocopper but which is rich in ammonium bisulf-ate, is transferred toevaporator 22 Where a substantial amount of its water is removed. Thewater vapor is condensed in condenser 24 yand passed to a water storagetank 26. The ammonium bisulfate concentrate from evaporator 22 may then'be passed to the vacuum crystallizer 4 or directly to the anodecompartment 8 of the diaphragm cell 10. `In either case, the-ammoniumbisulfate hows to the anolyte as supplementary sulfate values; however,it is preferred to pass the concentrate' into the vacuum crystallizer 4along with spent etchant in order to increase the eiiiciency of copperremoval from the etchant during crystallization in vacuum crystallizer4.

In the above illustrated process no chemicals need be added to thesystem after it has been put into operation, except for small operatinglosses, e.g., such as loss of chemicals in solutions adhering to etchedprinted circuits. Further, no by-products or Waste materials areproduced. The net effect of this systems operation is solely themovement of copper metal from the printed circuit board onto the coppercathode. All other chemicals remain in balance, and the appliedelectrical current merely reconverts sulfate values to persulfatevalues. The system defined in the drawing is exemplified in Example 1wherein each unit operation is carried out in fa batchwise manner beforebeing passed on to the next unit operation.

In operating the above system properly so that the copper plated at thecathode is equivalent to the quantity of persulfate produced at theanode, it is sometimes necessary to continue electrolysis Ibeyond thepoint at which al1 of the copper has -been deposited lfrom thecatholyte. This can be minimized by adjusting the solute concentrationsin the electrolytes.

In the embodiment of the above illustrated process which is `describedin Example l the process has been carried out 'by etching batchwise witha quantity of etchant until all of the etchant is spent and thentreating the entire etchant in each of the unit operations in sequenceuntil the etchant is finally regenerated. However, it is within thecontemplated scope of this invention to withdraw continuously only aportion of the etching solution in a constant stream and to regeneratethis stream of withdrawn etchant constantly by continuous treatment ineach unit, namely, the vacuum crystallizer, centrifuge and diaphragmcell. The regenerated stream of withdrawn etchant is then recycledcontinually to the etching machine.

Operation in such a completely continuous manner has the advantage thateach of the units, and most importantly the etching machine, would be incontinuous operation and the concentration of solutes in the etchantcould be maintained constant. This would result in highly desirableconstant etching rates and reduce the amount of maintenance required tokeep the etching precise'and within commercial tolerances. This systemwould also lend itself to automation most readily since it is a balancedsystem in which the chemical solutions would be recycled throughout theunits without varying in solute concentrations at any point in thesystem.

A process for regenerating anetching solution was carried out :asfollows. An etchant solution was prepared containing 1.1 moles ofammonia persulfate per liter and 5 p.p.m. of dissolved mercury as HgCl2which is the etching catalyst. This was. used to etch photo-resisted,single-sided printed circuit test panels, -the copper foil of whichweighed l oz. per sq. ft. The etching was conducted 'by immersing thetest panels in the air-agitated etchant at a temperature of 35-39 C.Etching rates and the quality of etch. were determined frequently duringthe run and were found to be satisfactory. When the etch rate droppedIbelow about 0.2 mil of copper per minute, the etchant was considered tobe spent. At this point one liter of it contained:

. Mole Persulfate .61

Dissolved copper a .45 Sulfate values .94

To one liter of this spent etchant .55 mole of NHJHSO.; was added. Theresulting solution was cooled to 2-5v C.

CUSO4 2504 26H20 Analysis indicated that about .26 mole had formed. Thecrystals were dissolved in water. Both the solution of the crystals andthe mother liquor were analyzed with the following results:

Mother liquor, mole Crystal solution, rnole Persullate Dissolved copperSulfate values The mother liquor was added to the anode compartment of adiaphragm cell; urea w-as 'added to give a .02% solution. The crystalsolution was passed into the cathode compartment of the same diaphragmcell. A platinum wire was suspended in the anode compartment to servelas an anode, while copper tu'bing was used as a cathode in contact withthe crystal solution in the cathode compartment of the cell. The anodeand thode were connected to a rectifier and electrolysis was conductedfor about 11 hours using a current of about 3 amperes. The currentdensity was about 1100 amps/sq. ft. at the anode and about 30 amps/sq.ft. at the cathode. The temperature of the anolyte and catholyte wasabout Ztl-25 C. and 18-20o C., respectively, during electrolysis. Afterelectrolysis the anolyte Iand catholyte were analyzed with the followingresults:

Catholyte, mole Anolyte, mole Persulate 87 None Dissolved coppe .21Trace Sulfate values 42 56 v Mole Persulfate .61

' Dissolved copper .47 Sulfate value .95

Quantities of materials noted above in the example have been correctedlfor amounts removed on analytical samples.

In the above example the amounts of ingredients found in the reusedetchant and in the exit catholyte were considered to correspond closelyenough to those in the spent etchant and bisulfate additive used in theprevious regeneration to assure that any number of additionalcrystallization, separation and electrolysis cycles could be run usingsubstantially the same amounts of materials in each step and obtainingsubstantially the same results.

EXAMPLE 2 A series of runs was made, in which photo-resisted,single-sided, 1 oz./sq. ft., copper laminated printed circuit testpanels wereretched. In these runs 500 ml. portions of a fresh aqueousetching solution, each having an ammonium persulfate concentration ofabout 1 M and 5 p.p.m. of HgCl2 (which is used as the etching catalyst),were used to etch the abovementioned circuit test panels until thesolutions were about 50% depleted of their `ammonium persulfate. Theetching was conducted by immersing the test panels in the air-agitatedetchant maintained at a temperature of P18-39 C. The average etchingrates were on the order of from about 0.25 to about 0.30 mils of copperper minute and the quality of etch obtained was good. -In general, anetch rate of about 0.2 mil of copper per minute is consideredacceptable'for commercial use.

The spent etching solutions were cooled to a temperawas suspended in thecup and served as the anode while a copper tubing 0.25 in. in diameterwas Wrapped in a helix around the porous cup and served as the cathode.Water was passed through the copper tubing for better temperaturecontrol. The anolyte was continually agitated by means of aTeflon-coated magnetic stirring bar placed in the base of the porcelaincup. The anode and cathode were connected to a rectifier which supplieddirect current. The electrolytic cell was operated 'by adjusting thevoltage passing through the cell to give the desired current. Each ofthe solutions in R-uns 1-4 was subjected to electrolysis for 60 minutes.

The ammonium persulfate contents of the anolyte solutions both prior toand subsequent to regeneration were determined by addition of a knownexcess of stand-ard ferrous ammonium sulfate and 'back titration withstandard potassium permanganate. The compositions of the anolyte andcatholyte prior to electrolysis are given in Table I along with thefinal persulfate contents of the anolytes. The conditions under whichelectrolysis was carried out are also set forth in Table I. `In Run 3,sulfuric acid was ladded to the anolyte to supply additional sulfatevalues for conversion to persulfate. In Run 4, 0.02% by weight of ureawas added to the anolyte and catholyte to increase the anode efficiencyfor production of persulfate.

The regenerated anolyte solution from Run 4 was used as a regeneratedetchant to etch test panels identical to those etched by the initialetchant. The rate of etch was found to be substantially above 0.2 mil ofcopper per minute and the quality of etch was good.

TABLE I l Final Current Density, Temperatures, C. Electrode Run InitialElectrolyte Compositlon (CNHtmgOfs Amps Volt Amps/Ft.2 Efficiency,Percent O11 en 0 Anolyte Catholyte Auolytc Anode Cathode .AnolyteCathode Anode Cathode 0.48 M (NHQQSQOS 1 0.5 M (NH4)2SO4. 0.5 M(NH4),SO4 0.61M 3 6.2 to 8.2 1,150 37 25 18 23 96.0

0.5 M CuSOi 0.5 M CuSOi. 0.46 M (NHDQSiOs l 2 0.5 M (NH4)2SO4 0.5 M(NH4)1SO4 0. 58M 3 5 to 5.8 1, 150 25 26 22. 5 22 84. 6

0.5 M ons 0.5 M ous0... -.l 0 M 'mvr'isi'fs" 0 .5 l i 2 4. 3 0. 5 MCus()4 0.71M 3 5 to 5.4 1,150 37 23 18 l. lVI HQSO; 0'5lyi[ r(\INHH0%OS"0"r`Eii` 's`-' 0.51 i .5r iz 4. 4 0.5 M CUSLUN' 0.5 M CuSOLU 0.74M3 5.5 to 7.0 1,150 25 27 22 40 91.6

.02% urea .02% urea ture of about 2-3 C. using a vacuum typecrystallizer EXAMPLE 3 operating at a vacuum of about 5 mm. of mercury.Blue crystals containing copper sulfate and ammonium sulfate valuesprecipitated lfrom the solutions; these crystals were identified asCUSO4(NH4)2SO4-6H2O. The crystals were filtered from the mother liquorand suicient water was added to the crystals to makea solution of 0.5 Mcopper sulfate and 0.5 M ammonium sulfate. These solutions were used asthe catholytes in subsequent electrolytic regenerations. Water was addedto the mother liquor to replace that removed during the Vacuumcrystallization until solutions containing about 0.5 M ammoniumpersulfate were obtained; these were used as the anolytes in theelectrolytic cell during the regeneration process.

The electrolytic cell comprises a porous porcelain cup standing in alarger glass beaker. The porous cup contained the anolyte and the areabetween the cup and the glass beaker contained the catholyte. The porouscup served as the diaphragm to retard diffusion of persulfate from theanolyte (contained in the cup) to the catholyte (contained in an annularspace between the cup and the vglass beaker). A platinum wire 0.04 in.in diameter A series of runs were made in substantially the same manneras set forth in Example 2, Run 1, except that in place of copper thespent etchant contained either cobalt, iron, nickel or zinc dissolved inthe etchant. The spent etchant containing the ingredients andconcentration level set forth in Table II, were cooled to a temperaturegiven in Table Il using a vacuum type crystallizer. Crystals wereprecipitated having the values set forth in Table II for each of thesolutions. The crystals were filtered from the mother liquors andsufficient water was added to these crystals to dissolve them. Theresulting solutions were then used as the catholytes in subsequentelectrolytic regenerations. The mother liquors which remained aftervacuum crystallization were used as the respective anolytes in the aboveelectrolytic regenerations. After operation of the electrolytic cells asset forth in Example 2, Run l, the anolytes were analyzed to determineif there was an increase in persulfate concentration, while the cathodicchamber was observed to determine if the etched metal plated out on thecathode. The results are given in Table II.

- A TABLE Il Spent Etchant Composition Eloctrolyzed Solutions Metal(Molar Concentration) Crystallizing Crystallizcd Values Hun EtchedTemperature NHiSzOs (NI-M2804 Metal Sulfate Cathode Anode 1 Cobalt.- 0.5 0. 5 0.5 Cobalt sulfate.. 0 to 2 C Cobalt sulfate, Cobalt metalPnrsulfate values increased ammonium sulfate. deposited. 'inconcentration. 2 Iron 0.5 0.5 0.5 Iron sultatc 3 C Iron sulfate,ammonium Iron metal Do.

v sulfate. deposited. 3 Nickel.. 0.2 0.2 0.2 nickel sulfate-.. 0 to 2 CNickel sulfato, ammon- Nickel metal Do.' 4

ium sulfate. deposited. 4 Zinc..." 0. 5 0. 4 0.4 Zinc sulfatc 0 to 2 CZinc su1fatc,-ammonium Zinc metal` 1)0.

-sulfate. deposited.

Pursuant to the requirements of the patent statutes, the principle ofthis invention has been explained and exemplified in a manner so that itcan be readily practiced 4by those skilled in the art, suchexemplification including what is considered to represent the bestembodiment of the invention. However, it should be 'clearly understoodthat, within the scope of the appended claims,

the invention may be practiced by those skilled in the art,

and having the benefit of this disclosure otherwise than as specificallydescribed and exemplified herein.

What is claimed is:

1. A process for regenerating an ammonium persulfate etching solutionwhich contains a dissolved metal selected from the group consisting ofcopper, cobalt, iron, nickel and zinc which comprises separating fromsaid solution a persulfate-free mixture containing ammonium sulfatevalues and metal sulfate values corresponding to said dissolved metalland leaving a remaining solution containing sulfate values, adding saidpersulfate-free mixture into the cathode section of an electrolytic celly.for use as a catholyte, adding said remaining solution containingsulfate values into the anode section of said electrolytic cell for useas an anolyte, said cathode and anode sections of said electrolytic cellbeing contained in a manner to prevent substantial amounts of persulfatevalues present in the anolyte from mixing with the catholyte, butpermitting the free passage of at least hydrogen ions between saidanolyte and said catholyte, passing an electric current through saidcatholyte and anolyte by means of -a cathodic electrode contacting saidcatholyte and an anodic electrode contacting said anolyte, convertingsulfate ions to persulfate ions at said anodic electrode, converting thedissolved metal ions to the corresponding metal at said cathodicelectrode, and recovering an anolyte solution having an increasedpersulfate content.

2. The process of claim 1 wherein the dissolved metal is copper.

3. The process of claim 1 wherein the dissolved metal is zinc.

`4. The process of claim 1 whe-rein the dissolved metal is iron.

5. Process of claim 1 lwherein urea in amounts of from about 0.005% to0.20% is added to said anolyte.

6. Process of claim 1 wherein ammonium thiocyanate in amounts of fromabout 0.005% to 0.20% is added to said anolyte.

7. Process of claim 1 wherein said electrolytic cll contains a`diaphragm separating said anolyte and catholyte solutions.

8. A process for lregenerating an ammonium persulfate etching solutionwhich contains a dissolved metal selected from the lgroup consisting ofcopper, cobalt, iron, nickel and zinc which comprises cooling saidsolution to crystallize metal sulfate values corresponding to saiddissolved metal and ammonium sulfate values from said solution withoutcrystallizing substantial amounts of ammonium persulfate, separating thecrystallized solids from the remaining mother liquor, adding saidcrystallized solids to an aqueous liquor to make up a substantiallypersulfatefree mixture, adding said persulfate-free mixture into thecathode section of an electrolytic cell for use as a catholyte, addingthe -remaining mother liquor containing sul- `fate values into the anodesection of the electrolytic cell for use as thev anolyte,lsaid cathodeand anode sections of said electrolytic cell being contained in a'manner to prevent substantial amounts of persulfate values present insaid anolyte from mixing with said catholyte but permitting the freepassage of at least hydrogen ions between said anolyte and saidcatholyte, passing an electric current through said catholyte and saidanolyte by means of a cathodic electrode contacting said catholyte andan anodic elect-rode contacting said anolyte, converting the dissolvedmetal ions to the corresponding metal .at said cathodic electrode,converting sulfate ions to persulfate ions at said anodic electrode andrecovering an anolyte solution having an increased persulfate content.

9. Process of claim 8 wherein the dissolved metal is copper.

10. Process of claim 8 wherein vthe dissolved metal is zinc.

11. Process of claim 8 wherein the dissolved metal is iron.

12. Process of claim 8 wherein said cooling is carried out attemperatures of from about 0 to 20 C.

13. Process of claim 8 wherein said cooling is carried out attemperatures of from about 0 to 10 C.

14. Process of claim 8 wherein said electrolytic cell contains adiaphragm separating said anolyte and catholyte solutions.

15. Process of claim 8 wherein urea in amounts of from about 0.005% to.20% are added to said anolyte.

16. Process of claim 8 wherein ammonium thiocyanate in amounts of fromabout 0.005% to 0.20% are added to said anolyte.

17. A process for 'regenerating an ammonium persulfate etching solutionwhich contains dissolved copper which comprises mixing said etchingsolution with an aqueous ammonium bisulfate solution, cooling theresulting Solution to crystallize copper sulfate and ammonium sulfatevalues from said resulting solution without crystallizing substantialamounts of ammonium persulfate, separating the crystallized soli-dsyfrom the remaining mother liquor, adding said crystallized solids to anaqueous liquor to make up a'substantially persulfate-free mixture,adding said persulfate-free mixture into the cathode section of anelectrolytic cell for use as the catholyte, adding the remaining motherliquor containing sulfate values into the anode section of theelectrolytic cell for use as the anolyte, said cathode and anodesections of said electrolytic cell being contained in a manner toprevent substantial amounts of persulfate values present in said anolytefrom mixing with said catholyte, but permitting the free passage of atleast hydrogen ions between said anolyte and said catholyte, passing anelectric current through said catholyte and said anolyte by means of acathodic electrode contacting said catholyte and an anodic electrodecontacting said anolyte, converting copper ions to metallic copper atsaid cathodic electrode and forming ammonium bisulfate in saidcatholyte, converting sulfate ions to persulfate ions at said anodicelectrode, passing the electrolyzed catholyte into an evaporator toremove water therefrom, yrecovering a concentrated ammonium bisulfatesolution from said evaporator, and recovering an anolyte solution havingan increased persultate content and suitable as an ammonium persulfateetching solution.

18. Process of claim 17 wherein said electrolytic cell 13 contains adiaphragm separating said anolyte and catholyte solutions.

19. A process for regenerating an ammonium persulfate etching solutionwhich contains dissolved copper which comprises cooling a first portionof said solution to obtain a rst crystal precipitate containing coppersulfate values and ammonium sulfate values substantially lfree ofammonium persulfate, separating said first crystal precipitate from theremaining first mother liquor, adding said iirst crystal precipitate toan aqueous liquor to Imake up a substantially persulfate-free firstmixture, adding said persulfate-free first mixture into the cathodesection of an electrolytic cell for use as a catholyte, adding said rstmother liquor containing sulfate values into the anode section of.

said electrolytic cell for use as the anolyte, said cathode and anodesections of said electrolytic cell being contained in a manner toprevent substantial amounts of persulfate values present in said anolytefrom mixing with said catholyte but permitting the free passage of atleast hydrogen ions between said anolyte and said catholyte, passing anelectric current through said catholyte and said anolyte yby means of acathodic electrode contacting said catholyte and an anodic electrodecontacting said anolyte, converting copper ions to metallic copper :atsaid cathodic electrode and forming ammonium bisulfate in saidcatholyte, converting sulfate ions to persulfate ions at said anodicelectrode, recovering a irst anolyte solution having an increasedpersulfate content, recovering a rst catholyte solution containingammonium bisulfate and having a decreased copper content, recycling saidirst anolyte solution to said ammonium persulfate etching solution,dissolving additional copper with said ammonium persulfate etchingsolution, withdrawing a second portion of said solution and cooling saidsecond portion of said solution to obtain a second crystal precipitatecontaining copper sulfate values and ammonium sulfate valuessubstantially free of ammonium persulfate, separating said secondcrystal precipitate from the remaining second mother liquor, adding saidsecond crystal precipitate to an `aqueous liquor to make a substantiallypersulfate-free second mixture, adding said persulfate-free secondmixture to said cathode section of an electrolytic cell for use as thecatholyte, passing said second mother liquor containing sulfate valuesand said iirst catholyte solution containing ammonium bisul-fate intothe anode section of said electrolytic cell for use as the anolyte, saidcathode and anode sections of said electrolytic cell being contained ina manner to prevent substantial amounts of persulfate values in saidanolyte from mixing with said catholyte but permitting the tree passageof hydrogen ions from said anolyte to said catholyte, passing anelectric current through said catholyte and anolyte by means of acathodic electrode contacting said catholyte and an anodic electrodecontacting said anolyte, converting copper ions to copper at saidcathodic electrode and forming `ammonium bis-ulfate in said catholyte,converting sulfate ions to persulfate ions at said anode, recovering asecond anolyte solution having an increased persulfate content,recovering a second catholyte solution containing ammonium bisul-fateand having a decreased copper content for use in supplying sulfatevalues to anolyte solutions of succeeding regeneration cycles, andrecycling said second anolyte solution to said ammonium pers'ulfateetching solution for additional etching.

20. Process of claim 19 wherein said cooling is carried out at atemperature of from about 0 to 20 C.

21. Process of claim 19 wherein urea in amounts of from about 0.01% to0.20% is added to said anolytes.

22. Process of claim 19 wherein ammonium thiocyanate in amounts of fromabout 0.01% to 0.20% is added to said anolytes.

23. Process of claim 19 wherein said electrolytic cell contains adiaphragm separating said anolyte and catholyte solutions.

References Cited UNITED STATES PATENTS 2,435,714 2/ 1948i Fusco et al204-237 XR 2,589,982 3/1952 Wood et al. 204-82 3,256,165 6/1966 Williams204-143 JOHN H. MACK, Primary Examiner.

D. R. JORDAN, Assistant Examz'ner.

1. A PROCESS FOR REGENERATING AN AMMONIUM PERSULFAATE ETCHING SOLUTIONWHICH CONTAINS A DISSOLVED METAL SELECTED FROM THE GROUP CONSISTING OFCOPPER, COBALT, IRON, NICKEL AND ZINC WHICH COMPRISES SEPARATING FROMSAID SOLUTION A PERSULFATE-FREE MIXTURE CONTAINING AMMONIUM SULFATEVALUES AND METAL SULFATE VALUES CORRESPONDING TO SAID DISSOLVED METALAND LEAVING A REMAINING SOLUTION CONTAINING SULFATE VALUES, ADDING SAIDPERSULFATE-FREE MIXTURE INTO THE CATHODE SECTION OF AN ELECTROLYTIC CELLFOR USE AS A CATHOLYTE, ADDING SAID REMAINING SOLUTION CONTAININGSULFATE VALUES INTO THE ANODE SECTION OF SAID ELECROLYTIC CELL FOR USEAS AN ANOLYTE, SAID CATHODE AND NODE SECTIONS OF SAID ELECTROLYTIC CELLBEING CONTAINED IN A MANNER TO PREVENT SUBSTANTIAL AMOUNTS OF PERSULFATEVALUES PRESENT IN THE ANOLYTE FROM MIXING WITH THE CATHOLYTE, BUTPERMITTING THE FREE PASSAGE OF AT LEAST HYDROGEN IONS BETWEEN SAIDANOLYTE AND SAIID CATHOLYTE, PASSING AN ELECTRIC CURRENT THROUGH SAIDCATHOLLYTE AND ANOLYATE BY MEANS OF A CATHODIC ELECRODE CONTACTING SAIDCATHOLYTE AND AN ANODIC ELECTRODE CONTACTING SAID ANOLYTE, CONVERTINGSULFATE IONS TO PERSULFATE IONS AT SAID ANODIC ELECTRODE, CONVERTING THEDISSOLVED METAL IONS TO THE CORRESPONDING METAL AT SAID CATHODICELECTRODE, AND RECOVERING AN ANOLYTE SOLUTION HAVING AN INCREASEDPERSULFATE CONTTENT.